This invention relates to plastic tubing and, more particularly, this invention relates to webbed plastic tubing. The webbed plastic tubing is particularly useful as a support for a filtering medium, although there are numerous other uses such as protective sleeves for machined shafts during shipping; decorative items like candle holders, lamp shades, and packaging material; protective coverings for seedling trees; and the like. Examples of the use of the webbed plastic tubing as packaging material are onion bags, potato bags, etc. As a protective cover for seedling trees, the tubing is particularly useful in reforestation. The seedlings are covered with the webbed tubing made of polypropylene. The tubing protects the seedlings from animals, yet admits enough light for growth. Since the polypropylene is photodegradable, the cover deteriorates by the time the tree is self-sustaining.
The preferred use, however, is as a support for filter media and the following description will refer thereto, although it should be distinctly understood that the invention is not limited thereto. In a typical installation, there are one or more cartridge filters disposed in a pressure vessel, each one being 21/2 to 3 inches in diameter and 10 to 60 inches long. In each cartridge housing there is a tubular support and a filter on the support. Typically the filter medium or pad is a needled polypropylene felt.
In many types of filtering systems it is necessary to provide a support for the filter medium which is rigid yet permeable. In installations where a flat support is called for, it is fairly simple to provide a foraminous material such as screening. In other types of installations, supports of a more complex shape are needed. The support is often tubular in shape for use in cartridge filters. Such tubular supports are often made by perforating a plastic pipe or tube. Since this is usually a manual operation, it is expensive to make.
Another method of making tubular supports involves extruding a plurality of separate strands and crossing the strands at the diehead of the extruder to form a porous tube which resembles a woven product. The cost of making such a product is also rather high.